Prestressed concrete pipe (PCP) consists of a concrete pipe that is prestressed using high strength prestressing metal wires wound around the concrete pipe to approximately 70% of yield strength. The concrete pipe may or may not also have longitudinal pre-stressing strands, and may or may not also have a steel cylinder embedded within or lined with concrete. A PCP is designed to operate under pressure in such a way that the concrete core remains under compression for all operating conditions of the pipeline. The design of individual sections of the PCP along the length of a pipeline may vary as the operating pressures change.
Breaks in the pre-stressing wires, such as caused by corrosion or hydrogen embrittlement, result in a localized loss of prestress in the concrete core, which can lead to a sudden rupture of a pressurized pipeline. It is therefore beneficial to be able detect broken prestressing wires in a given section of pipe, such as through periodic inspections of the pipeline
In addition, pipeline contractors have been known to install sections of pipe in an incorrect order, resulting in a weaker pipe experiencing high pressures which exceed its design. It is therefore also beneficial to be able to know if the right pipe section has been laid in the right location.
One method of non-destructive testing of pipe is called Remote Field Testing (RFT). A RFT probe consists of an exciter coil which sends a signal to a detector coil, although there may be more than one detector coil, may be more than one exciter coil, or exciters and/or detectors other than coils may be used. The exciter coil is energized with an AC current and emits a magnetic field. In the case of pipe in which a metal cylinder is embedded in concrete, the magnetic field travels outwards from the exciter coil, through the pipe wall, and along the pipe. The detector coil is placed inside the pipe two to three pipe diameters away from the exciter coil and detects the magnetic field that has traveled back in from the outside of the pipe wall (for a total of two through-wall transits). In areas of metal loss, the field arrives at the detector with a faster travel time (less phase lag) and greater signal strength (amplitude) due to the reduced path through the steel.
In PCPs, the prestressed wires are located away from the inner surface of the pipe, and an RFT probe can be used to detect breaks in prestressed wires. U.S. Pat. No. 6,127,823, issued Oct. 3, 2000, discloses such a method in which the pipe is a PCP. In PCPs, there is a transformer coupling (TC) effect, in which the magnetic field produced by the exciter coil induces a current in the prestressing wires wound around the pipe, which in turn induces a current in the detector coil. In a PCP in which a metal cylinder is embedded in the concrete wall of the pipe, the signal detected by the detector coil is therefore a combination of a signal induced by the TC effect and a signal propagated along the outside of the pipe. Using anomalies in this detected signal, abnormalities such as prestressing wire breaks can be detected.
The receiver coil is spaced far enough away longitudinally from the exciter coil that direct field effects are negligible. Magnetic shielding may also be used between the exciter coil and the detector coil to further reduce direct field effects. The wide spacing is necessary so that the detector coil is within the remote field zone of the exciter coil in order that the detector coil only (or at least predominantly) detects fields that are transmitted through the pipe and that interact with the prestressing wires. The use of two coils may require more time to set up the apparatus. An operator must also ensure that there is no coupling between the transmitter and the receiver, or at least that such coupling is minimized. The detected signal may also include spurious signals resulting from movement of the coils relative to each other or relative to the pipe axis.
Impedance probes offer another way of performing non-destructive testing of pipe in order to detect flaws. This is often referred to as Eddy-current Testing (ECT). In ECT a small circular coil, typically much less than one to two inches in diameter and much less than three inches long, carrying current is placed in proximity to the pipe, which must be electrically conductive. The alternating current in the coil or coils generates a changing magnetic field which interacts with the conducting pipe and induces eddy currents within the metal cylinder embedded in the pipe wall. Variations in the phase and magnitude of these eddy currents can be monitored using a second receiver coil, or by measuring changes to the current flowing in the primary excitation coil if a single coil is being used. Variations in the electrical conductivity or magnetic permeability of the pipe, or the presence of any flaws, will cause a change in eddy current and a corresponding change in the phase and amplitude of the measured current. ECT can detect very small cracks in or near the surface of the pipe, such as corrosion pitting or cracking.
ECT does have several limitations, however. Only conductive materials can be tested. In general, the surface of the material must be accessible, and the depth of penetration into the material is limited by the conductivity and permeability of the metal lining, as well as the operating frequency of the ECT probe. This makes conventional ECT probes impractical for detecting breaks in prestressing wire located on the far side of metallic cylinders. Even if no metal cylinder is present, several inches of concrete between the ECT probe and the prestressing wires will create a large lift off between the probe and the wires. This greatly reduces the capability of conventional ECT probes in analyzing prestressing wires. For these reasons, impedance probes are not used for inspecting for breaks in prestressing wires in PCPs.